Six months ago, Karukozaka High School had vanished into the Underworld. Upon its reemergence, all 560 students and 28 faculty members remained missing. Treated as a bizarre case of mass disappearance, the entire ordeal was forgotten, but nobody noticed when two of the students had returned. These students are Nobu—a boy with an uncanny ability to sniff out demons—and Yumi Shirakawa. Wishing to move on from the trauma, Nobu and Yumi transfer to different schools and lose contact with each other, but they both remain vigilant of demonic activity. Unfortunately, the portal to the Underworld resurfaces at the location of their former school, causing two unique factions to seek out survivors of the Karukozaka incident. The Ring of Gaea seeks to enact martial law within Tokyo in order to contain the demonic threat. On the other side, the Order of Messiah sees Nobu and Yumi as threats to the new world they wish to establish. When his mother is killed in front of his eyes by a demon, Nobu realizes that he cannot run away from confronting his demon-infested past. [Written by MAL Rewrite]